This invention relates to a plasma arc torch which can be used in a variety of plasma arc metal working processes including cutting, welding and other metal surface treatments.
Plasma arc torches operate by directing a plasma jet arc formed of ionized gas particles toward a workpiece. In the typical plasma torch operation, a gas to be ionized is injected within the plenum cavity of a torch tip around a charged electrode. The plasma then is directed through an arc constricting orifice formed in the tip. A sufficiently high voltage is applied to the torch tip to cause a spark to jump across the electrode and torch tip to heat and create an ionized gas column which issues from the arc constricting orifice as a flame. As the torch head is moved toward a workpiece, a transferred arc jumps from the electrode to the workpiece since the impedance of the current path in the workpiece is lower than the impedance through the welding tip current path. Thus, the ionized particles carried through the tip arc constricting orifice will form a lower resistance path for establishing the main arc to the workpiece.
The purpose of the arc constricting orifice is to project the arc as part of the flow of gas in the shape of a long, laminar or columnar flame. Constricting the arc also tends to accelerate the rate of gas flow as the arc energy is focused as a long, laminar or columnar flame on a workpiece.
The arc will pass from the electrode through the tip arc constricting orifice to the workpiece, as long as that route represents the lowest resistance path. Sometimes, there is a lower resistance path to ground through the torch tip, and this causes the phenomenon known as double arcing. When double arcing occurs, the metallic torch tip forms part of the current path to ground, thereby creating two arcs, one from the electrode to the tip, and the second from the tip to ground. Unfortunately, double arcing results in damage to the tip due to metal melting at the anode and cathode portions of the structure from which the arcs form. The usual causes of double arcing are touching the tip to a workpiece, using two high an arc current for a given tip orifice diameter or using too low a gas flow for a given current.
While it is generally true that plasma arc torches create a long laminar or columnar flame issuing from the arc constricting orifice, what is not generally known is that the typical torch tip is constructed to let the plasma arc expand as it comes out of the arc constricting orifice, thereby causing the distance from the torch tip to the workpiece to be changed. This uncontrolled plasma arc expansion and unintended change in the length of the long, laminar or columnar flame issuing from the arc constricting orifice can also produce double arcing, damaging the torch tip as well as the workpiece. It has also been discovered that the arc established from the electrode may vary, depending on the lowest resistance path. Thus, uniformity and consistency in establishing an arc from the orifice to the workpiece has not been possible with existing constructions.